Honestly, this is the most viable moveset. Just a quick remark and a question:

Remark: I'd add 40 points into HP for an extra point of leftover recovery (if this is the intended item). You can also add an additional 4 to obtain an odd number to lessen entry hazard damage, which I chose to do.

Explanation: Hp stats for previously mentioned reasons. 200 Def and 252 SpD in attempt to even out defenses and, finally, 8 Spe IV, cause I assume these IVs have an intended use. Let me know about those IVs (if you ever get this. New here and don'y know if people get notifications)

The only other thing I would add, is an option for Light Screen instead of Substitute. Substitute can be hard to obtain especially in ORAS. Light Screen cuts Sp Attack in half, which is a good alternative.

These move set questions are here just to be able to have a whole database of different move sets for every pokemon. Pokemaster has a set that he wanted to share, so he posted it as an answer. These questions are for the whole community, not just for Pokemaster.

I'm going to use my Whimsicott as a support when he's reach level 100, and these are the moves I plan to use. He wasn't specially bred or anything, but unfortunately I wasn't able to give him encore, but I'd prefer it instead of Leech Seed.

Ability: Prankster
Toxic (To help wear the enemies down)
Substitute (to stop Whimsicott from getting attacked)
Leech Seed (To revive Whimsicott's HP after Substitute)
Psychic/Energy Ball (Haven't decided yet, Energy Ball is a good special grass attack for a grass pokemon, but Psychic will be strong against fighting/poison types which tend to do a lot of damage to my team)

encore leech seed energy ball and tailwind go with a timid nature and i guess put his ev's in both defenses and in speed but watever he is kinda known for being an annoying staller but sets up the team with tailwind pretty well

So what I do is send Whimsicott against a Physical Attacker and use Cotton Guard twice so his defense will be really high and he'll be able to take many hits. Next, I use Substitute and then Leech Seed so he'll get Leftovers and Leech Seed which is awesome. For the last move there are a few options Giga Drain for STAB and health regeneration with Leftovers and Leech Seed which is super duper or HP Rock for coverage and last but not least Toxic to slowly but painfully kill your opponent with Leech Seed. I like using Giga Drain just in case I get Taunted but the other moves work well too.

Fun in the Sun Sweeper

Growth Boosts your S.Attack(and Attack but that is not important here) by two stages in the sun. Solar-beam is very Strong STAB. Psychic is for coverage. HP(Fire) is for coverage and is boosted by the sun.

Impenetrable Physical Wall.
Switcheroo burns, while Cotton Guard raises Def to the max in 2 turns, while taunt destroys other taunters when they switch in. Charm is to lower Atk even further, or Giga Drain provides a way of healing, and makes you not a total taunt bait.

Sacrifice

This is a great Pokemon to use to set the stage for a Stat booster or a Pokemon that makes use of Subs. Taunt prevents the foe from setting up any hazards or statuses that could potentially foil the strategy. Stun-spore is great for crippling and it works on all types. Encore one of the most annoying moves in the universe. And finally Memento you half the offense of the opponent then go to a Pokemon that can boost while your foe is busy swearing about his now nearly useless Pokemon. While normally after you use Memento the opponent will switch out but remember you still get in a boost on the turn they switch and they have a paralyzed Pokemon and did not get a chance to set up their hazards.

Taunt is a problem, but really.. it's predictable enough to switch out on.
Cotton Guard will pretty much always go first ( prankster ) so you're set for bulky defenses.
Leech Seed and Toxic will slowly kill your opponent ( while Leech Seed feeds you back HP )
Protect stalls out Leech seed, Toxic, AND Leftovers.

Only really meant for Tailwind support. But with Prankster, he does a marvelous job. His EVs allow his to take a hit whike setting up, then with Tailwind support, he is sure to U-Turn before they hit him unless they have priority. Toxic makes a fearsome pair with Prankster as well, cause if you wall the opponent, one Sub and a Toxic can turn the tables on them.

Mine mixes attacking and defending. I beat up a Snorlax with this pillow!

Moves:

Cotton Guard: Fluff the pillow!
Leech Seed: recover with Leftovers
Giga Drain: recover and attack at same time, nasty with high defenses and recover HP total of 3 times a turn max in single battles
Hurricane: for grass, who resists Giga Drain and immune to Leech Seed

Switcheroo can be used to permanently cripple mon's that don't have fling or switcheroo themselves. Encore is to come in and stop setup sweepers from setting up, and works well with taunt. Speaking of taunt, Whimsicott can completely shut down the likes of Sableye in particular, as it outspeeds and takes minimal damage from foul play thanks to the new fairy typing. If Taunt isn't your thing, or you already have a Pokemon with taunt, than endeavor is a cool option to do respectable damage when your health is low. U-Turn allows you to scout and provides momentum, however you can swap it out for endeavor if you prefer.

Most likely you're gonna wanna lead off with the stun spore, then go ahead and use the leech seed to get your health back, whimsicott is an excellent sweep breaker for sure, but not so great as a lead considering most people lead off with the biggest threats at the beginning of the battle, once those are gone he's clear for take off.